The basic goal is to measure auditory speech perception capacity in aided and implanted children. Results will be used to establish efficacy of cochlear implants in children and to contribute to candidacy decisions. A key to this work is the use of outcome measures that are: a) maximally sensitive to sensory capacity, b) minimally sensitive to cognitive and linguistic status, and c) predictive of later development of sentence-level speech perception. Without such measures there is a clear danger that deaf children will receive implants that provide less auditory capacity than was already available by amplification. Previous work supports the conclusion that a measure of the ability to perceive phonetic contrasts, presented in a varying phonetic context, meets these requirements The tasks so far used to obtain this measure, however, require either that the child be at least 7 years old or, if younger, that basic speech skills are established. Aims for proposed research are: 1. to develop a behavioral test of phonetic contrast perception, suitable for prephonological 3 year old children, in which the child must detect and respond only to deviant utterances in a stream of repeated utterances 2. to develop an electrophysiological approach to obtaining the same information, using the same stimuli and the mismatch negativity response, 3. to validate the measure of phonetic contrast perception as a predicator of sentence-level speech perception in older aided and implanted children, 4. to measure the limits of personal hearing aids in terms of permitting full use of auditory capacity in severely and profoundly deaf children, 5. to determine, in implanted adults, the increase of auditory sensory capacity made possible by modified speech processing strategies developed under project #3. If successful, the results will provide valid data on the sensory consequences of amplification and cochlear implantation as treatments for severe and profound childhood deafness. They will also increase the objectivity of predictions of outcome, with and without implants, during discussions of individual candidacy.